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Manny Pacquiao's promoters said his prospective bout with Floyd Mayweather Jr. was dead late Wednesday night after a mediation session failed to resolve the fighters' differences over drug testing, scrapping what was likely to be the richest fight in boxing history.

The bout was slated for March 13 at the MGM Grand Garden in Las Vegas, but Top Rank said it couldn't reach an agreement with Golden Boy Promotions, which represented Mayweather in the negotiations, after nine hours of mediation Tuesday and more discussions Wednesday.

Although neither side was allowed to publicly discuss the specifics of their dispute, Mayweather apparently balked at a hard-fought compromise in the testing issues first raised by the former welterweight champion.

"I knew this was going to happen," said a weary Bob Arum, Pacquiao's promoter. "You had to play it out."

The promoters went into the lengthy mediation Tuesday to resolve their differences over drug testing for the fight. Both sides claimed every other detail for the fight had been decided, but Mayweather had demanded random blood testing in addition to unlimited urine testing, with Pacquiao balking at the stringent requests.

The fighters' representatives apparently thought they had a compromise after the mediation. The promoters were widely expected to formally announce the bout Wednesday -- but subsequent discussions with Mayweather during the day led Top Rank to declare the fight cancelled.

Arum was left fuming by Mayweather, who fought for Top Rank for several years.

"I've been saying this for years: He's a psychological coward who doesn't want to fight anybody who has a chance of beating him," Arum said. "He walked away from a rematch with Oscar [De La Hoya] that would have paid him a fortune because De La Hoya held him close in the first fight (in May 2007)."

After stellar pay-per-view numbers from their previous fights, both Pacquiao and Mayweather likely stood to make more than $25 million US from the welterweight bout. Mayweather returned to the ring after a 21-month absence in September with a victory over Juan Manuel Marquez, while Pacquiao (50-3-2, 38 KOs) pounded Miguel Cotto in November for his 13th straight victory since 2005.

Mayweather (40-0, 25 KOs) initially appeared eager to fight Pacquiao, widely thought to be the sport's pound-for-pound champion with his string of dynamic victories during Mayweather's aborted retirement. Both fighters quickly agreed to the initial points of a deal, with the fight scheduled for March so it wouldn't interfere with Pacquiao's run for political office in the Philippines.

But Schaefer then infuriated Top Rank by refusing to travel to Dallas for a meeting with Cowboys owner Jerry Jones, who offered to pay lavishly to hold the fight in his opulent new stadium.

Schaefer's actions, apparently at Mayweather's behest, meant the fight went to the MGM Grand in Mayweather's adopted hometown with no real competition.

After Mayweather went public with his requests for drug testing that went beyond the standards of the Nevada Athletic Commission, Pacquiao complicated the negotiations by filing a lawsuit last week alleging Mayweather and most of his camp's key players defamed him by falsely accusing him of using performance-enhancing drugs. Those confrontations led to the mediation, which apparently produced a solution acceptable to everybody — except Mayweather, according to Arum.

Arum said there's "no chance ever of salvaging it for March, no chance for it ever to happen." He plans to propose a mid-March bout with 154-pound champion Yuri Foreman to Pacquiao, who could become a champion in his eighth weight class.